Differences in crime rates among cities depend upon two ecological factors. Crime rates are often based on the population within a city's limits and do not consider the larger population of the metropolitan area which the city and its suburbs represent. Also, the impact of the city's facilities and economic activities on the entire metropolitan area plays a major role in determining the crime rate. For example, if only one city exists in the metropolitan area it will exert a greater attraction on nonresident crime participants than it would exert if one or more additional cities existed in the area. This theory was tested using data from Uniform Crime Reports, city demographic data, and the results of a travel to work study. While the population ratio was a necessary condition for crime, it was catalized by the existence of a dominance state. Neither the population ratio nor the dominance state correlated independently with the crime rate; however, when they were considered together, the two factors were closely related to crime rates. Moreover, the correlation between population ratios and crime rates increased as the level of city dominance increased. Tabular data and 19 references are included.